In analyzing the timing of an internal combustion engine, determination is made of the timing angle, that is, the number of degrees between the time a piston in a selected cylinder reaches its top-dead-center position and the time that combustion takes place in that cylinder. Automobile manufacturers commonly specify the timing angle at a specific engine speed as a compromise to numerous considerations which must be taken into account.
Electrical signals in response to the occurrence of the combustion event may be generated in several ways, such as in response to light during combustion in a diesel engine or in response to pulses of spark-plug igniting current in a spark-ignited engine.
The occurrence of top dead center is identified in the following manner. An internal combustion engine includes a wheel mounted on the end of the engine crank shaft so as to rotate therewith. A notch or other marking means is located in the periphery of the wheel. A receptacle for a magnetic probe is attached to the engine block and is so located that the marking means on the rotating wheel will pass the receptacle a known number of degrees of crank shaft rotation after the number one (or other selected) cylinder has reached its top-dead-center position. This known number of degrees is commonly referred to as "offset".
In certain instances, the receptacle is part of a sheet-metal bracket. Such brackets are not perfectly rigid and have a tendency to bend and deform as the magnetic probe is inserted and/or withdrawn. This tendency is worsened because the receptacle is often located in difficult-to-reach places.
The position of the receptacle is located so as to be spaced from top dead center precisely by the amount of offset. If the receptacle is moved in the manner indicated above, the magnetic signal will not occur at the correct instant; instead it would occur too late or too early, thereby giving rise to an erroneous measurement of timing angle.
It is important to ensure proper orientation in location of the receptacle. On the other hand, the mechanic who is tuning an engine does not want to waste time in adjusting the receptacle.